1. Field of the Invention
The present invention relates, in general, to the art of forming particles, and in particular, to the art of forming very small particles. Specifically, the invention relates to the formation of particles having a size of less than one micron in diameter.
2. Description of the Prior Art
The art generally defines ultrafine particles (UFP) as being particles having a diameter in the range of one nm to about one hundred nm (1.times.10-.sup.9 m to about 1.times.10-.sup.7 m). Such a particle is smaller than conventional fine powder and larger than a so-called atom cluster.
Ultrafine particles are of interest in the art because they have many applications. Examples of such applications include: production of powder catalysts, ceramics and electronics devices.
Specifically, ultrafine metallic particles have found use in the art of producing printed circuits and other such devices used in the construction of conductors, resistors, radiation and electrostatic shields which are required by many such devices. These applications of ultrafine particles use such particles in liquid suspension which is generally known in the art as "metallic inks".
Modern emphasis on miniaturization is dictating ever smaller and more precise detailing in these printed circuit components, and this has, in turn, spawned an industry of "micro-jet" printers which are controlled and guided by computers. Micro-jets spray very small globules of ink in the process of forming a component. It has been found that it is the size of the ink droplets that limits the miniaturization of the entire process because it is the size of the ink droplet that limits the size of the micro-jet sprayer.
Accordingly, there is a need for metallic inks having a droplet size that is as small as possible.
However, it is the size of the metallic particles that dictates the size of the ink droplets being sprayed. Accordingly, there is need for metallic particles that have a size smaller than the ultrafine particles presently used in the art.
The art presently has the capability to produce particles in the micron and submicron diameter sizes mentioned above with regard to the definition of ultrafine particles. For example, a Japanese project has produced such particles by a "smoke" process in which metals are vaporized into a low pressure gas, collected and then placed in liquid suspension. This process is limited to nonrefractory materials such as lead and gold and it is particularly dangerous to operate because of the high likelihood of a deflagration of the fine dry pyroforic powder produced. There is an acknowledged difficulty in capturing this dry powder and placing it in liquid suspension without re-agglomeration of the small particles into unusually large pieces. In addition, this is a rapid, high temperature process which is hard to control and which produces a wide range of particle sizes.
However, due to the just-discussed need for extremely small jets, even if the above-described drawbacks are overlooked these known processes still produce ultrafine particles that are too large for modern uses. Still further, such processes are not amenable to producing extremely small non-metallic particles.
Therefore, there is a need for a process that produces both metallic and non-metallic particles that are finer than the presently available ultrafine particles, yet does so in a manner that is safe and efficient.